This invention relates to an apparatus for improving the signal-to-noise ratio of image signals in a scan-type imaging system, and particularly to an apparatus for improving the signal-to-noise ratio with the intention of reducing noises on the signal waveform in the measurement of dimensions with a semiconductor surface pattern scanning electron microscope (SEM).
In the semiconductor manufacturing process, where fineness is ever upgrading, it is imperative to measure the dimensions of semiconductor surface patterns. However, lower acceleration voltages are used for observation for such reasons as reducing damage to the semiconductor caused by the electron beam and consequently much noises are included in the image signal. Therefore, in measuring the line width based on the scanning waveform, the signal-to-noise ratio of the signal must be improved as a preprocessing.
JP-A-59-112217, for example, describes the noise reduction techniques, which include the FFT (Fast Fourier Transform) method, the multiplication gradient method, the averaging method, and the multiple frame addition method. The FFT method reduces high-frequency components through the frequency analysis, and the multiplication gradient method and averaging method conduct the averaging process for adjacent pixels on the waveform of a scanning line to reduce noises in the image signal. The multiple frame addition method conducts more than one scanning for the same place so that noises are averaged out. It is generally known that addition of n frames improves the signal-to-noise ratio by .sqroot.n times, and this fact holds irrespective of the shape of object in observation.
Even though the FFT method, multiplication gradient method and averaging method suppress noises, the averaging of adjacent pixels results unfavorably in a faint scan image. The multiple frame addition method has a drawback of increased damage to the sample due to the iterative scan on the same place by the electron beam.